Exposure to a medical device which is implanted or inserted into the body of a patient can cause the body tissue to exhibit adverse physiological reactions. For instance, the insertion or implantation of certain catheters or stents can lead to the formation of emboli or clots in blood vessels. Similarly, the implantation of urinary catheters can cause infections, particularly in the urinary tract. Other adverse reactions to medical devices include cell proliferation which can lead to hyperplasia, occlusion of blood vessels, platelet aggregation, rejection of artificial organs, and calcification.
To reduce such adverse effects, pharmaceuticals, such as anticoagulants and antibiotics, have been administered in or on medical devices. A number of methods for delivering the drug(s) through implantation or insertion of the medical device involve covalently bonding the drug to the medical device, i.e, substrate. For example, U.S. Pat. No. 4,613,665 to Larm describes the coupling of heparin with reactive aldehyde groups to an animated surface by reductive amination.
Also, U.S. Pat. Nos. 5,112,457 and 5,455,040 to Marchant disclose the use of a similar approach to end-bind heparin on modified substrates. The substrate modification consists of depositing a film of plasma polymerized N-vinyl-2-pyrrolidone and attaching a spacer (e.g. PEG) on the film. The end group of the spacer is a primary amine, which can be bonded to aldehyde-ended heparin through reductive amination.
However, the covalent bonding approaches are limited. Only the surfaces of covalently bound coatings provide pharmaceutical activity, resulting in insufficient pharmaceutical activity at the treatment site. Furthermore, the drug loading of the medical device is limited by its surface area since the drug must be attached to the surface of the coating.
Pharmaceuticals have also been applied to medical devices by covering the surface with a coating containing them. A number of these coatings involve the ionic binding of the drug to the substrate. These approaches generally comprise the deposition of water-insoluble complexes of drugs and ionic surfactants on the surfaces of medical devices.
Illustrative of such approaches is the use of tridodecylmethylammonium chloride (TDMAC) or benzalkonium chloride, positively charged or cationic surfactants which are ionically complexed to negatively charged molecules of pharmaceuticals. Typical examples include tridodecylmethylammonium (TDMA)-heparin and TDMA-antibiotics. The former complex has been widely used as coatings on catheters, shunts and other blood contacting devices. The TDMA-heparin treatment can be applied to numerous biomedical materials including polyurethane, silicone, polypropylene, polycarbonate, PVC, metals and glass. TDMA-antibiotics have been used to reduce infections related to implants, urinary catheters and the like.
Although these ionic complex approaches allow numerous biomaterials to be coated with drugs without elaborate surface modification, they suffer from certain disadvantages. Notably, the ionically complexed drug tends to be quickly released from the medical device upon contact with body fluids so that its activity at the point of implantation or insertion diminishes rapidly. Attempts have been made to stabilize these coatings by crosslinking the ionically complexed drugs with glutaraldehyde or other bifunctional reagents. Recently, U.S. Pat. No. 5,441,759 to Crouther et al. discloses that exposure to gamma radiation and post-exposure heat treatment can strengthen the complex of TDMA-heparin to PVC surfaces. However, these attempts have demonstrated limited improvement. Specifically, such exposure to gamma radiation has been demonstrated in certain cases to have adverse effects upon the device. For instance, certain polymers degrade, crosslink, or change color upon exposure to gamma radiation, which may result in a loss of mechanical properties.
Also, attempts have been made to prolong the activity of ionically complexed drugs by mixing polymers with the drug-surfactant complexes to form coating compositions. See e.g. U.S. Pat. No. 5,525,348 to Whitbourne et al., U.S. Pat. No. 5,061,738 to Solomon et al., and U.S. Pat. No. 4,670,975 to McGary et al. However, inclusion of a polymer has not shown significant increase in prolonging activity. Moreover by employing drugs which are ionically complexed to surfactant, the amount of drug that can be loaded into the coating is limited since in general the drug constitutes only 20-50% of the complex. Thus the incorporation of the surfactant restricts the amount of drug that can be placed into the coatings of the medical device.
Hence, there is a need for stable coatings for medical devices which permit sufficient release of drugs at a certain rate or over a desired period of time into body fluid while maintaining high pharmaceutical activity on the surface. Therefore it is an object of the invention to provide such a coating for timed release of the incorporated drugs.
It is also an object of this invention to provide a drug-containing medical device which allows sustained delivery of the pharmaceutical or sufficient pharmaceutical activity at or near the coated surfaces of the devices.
Also, it is an object of the invention to provide medical devices with stabilized ionically complexed drug coatings and methods for making such devices.
Additionally, it is an object of the invention to provide a drug-releasing coating which adequately adheres to a medical device to allow the timed or prolonged application of the drug to body tissue.
It is a further object of the invention to provide methods for making a drug-releasing medical device which permit timed-delivery or long-term delivery of the drug.